Obscuration enamels are used on automotive glass to provide protection from ultra-violet rays which can deteriorate the adhesive glues that bond the glass to a vehicle body. Without such protection, such adhesive glues can degrade with exposure to the sun thereby compromising the integrity of the bond.
In addition to protecting the adhesive glues, obscuration enamels also conceal other automotive components, such as silver bus bars or antennae, from view from outside the vehicle. Pastes containing silver metal are often printed over the obscuration enamel prior to the firing and bending steps of producing automotive glass such as windshields, sidelites and backlites. Silver ions diffuse quickly at elevated temperatures of 400° C. and above. As a result, the silver ions migrate or diffuse through the obscuration enamel during the bending process when the automotive glass is fired and bent at a temperature ranging from about 600° C. to about 740° C. The silver migration results in a visible staining effect or discoloration in the obscuration enamel. The obscuration enamel, typically having a black color, turns a dark amber, gray or blue as a result of the silver migration.
Resistance to acid, typically from acid rain, includes another necessary quality of obscuration enamels. Previously, automotive manufacturers added polymeric protective molding or encapsulation of the glass edge to prevent acid rain from contacting the enamel itself. In recent years, the use of such methods has declined. Automotive companies have discovered that some enamels will deteriorate in the field when they are exposed to years of acid rain contact. This deterioration can exhibit itself as a change in color, i.e. black to gray, or, in very severe cases, complete removal of the enamel has been seen.
Accordingly, the industry is placing increased demand on higher performance obscuration enamels which possess superior chemical durability, scratch resistance and acid resistance. Further, the tendency of the obscuration enamels to stick to pressing molds during the glass forming process must be minimized or eliminated. During manufacturing, the glass often comes in contact with a pressing mold in order to shape the glass. This mold is usually covered with a woven stainless steel cloth or woven fiberglass cloth. It is important that the enamel has little or no tendency to stick to the cloth. It would be desirable to provide frits for use in high performance obscuration enamels.